selftests: add regression test for br_netfilter panic

Add a new netfilter selftests to test against br_netfilter panics when
VxLAN single-device is used together with untagged traffic and high MTU.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20241001154400.22787-3-aroulin@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>